The continual increases in global energy demand and greenhouse gas emissions call for more and more utilization of sustainable energy sources such as biomass. Among the existing biomass conversion technologies such as combustion, gasification and biochemical approaches, combustion is the most mature and widely used conversion technology. Several biomass-fired combined heat and power (CHP) plants with capacities in the order of 1000 kW e have been demonstrated in several European countries. However, biomass-fuelled micro-scale CHP (1--10 kW e ) suitable for domestic applications has yet to be commercialized or demonstrated. The micro-scale biomass-fired organic Rankine cycle (ORC) CHP system currently developed by the authors mainly consists of a biomass boiler, an ORC fluid evaporator, an ORC turbine, an alternator, a heat recuperator and a condenser. The boiler produces hot water which transfers heat to the organic working fluid via the evaporator. The generated organic fluid vapour drives a turbine to rotate an alternator, producing power. The expanded organic fluid vapour leaving the turbine transfers some of its heat to the recuperator and then is condensed by cooling water which can be heated to around 40°C for domestic washing and under-floor heating purposes. The preliminary testing of the CHP system has been carried out initially with a 9 kW electric boiler and then with a 25 kW th biomass boiler. The experimental findings of these preliminary tests are analysed and presented in the present paper. Copyright The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org, Oxford University Press.